1. Field of the Invention
The present invention relates to a method for manufacturing an active matrix substrate employed in a liquid crystal display device, and more particularly to a method for manufacturing a Thin Film Transistor (TFT) substrate provide with a Color Filter (CF).
2. Description of the Prior Art
In a conventional twisted nematic (TN) type liquid crystal color display device, a liquid crystal is interposed between a TFT substrate and a Color Filter (CF) substrate. In such a liquid crystal display device, a black matrix is generally provided on the CF substrate to prevent degradation of images to be displayed. Taking into account misalignment between the CF substrate and the TFT substrate, the black matrix has to be formed wide enough to securely prevent a light from leaking through the liquid crystal. Accordingly, an aperture ratio of liquid crystal display device becomes small and transmittance thereof becomes low.
As one of technologies for enlarging an aperture ratio to solve the above-stated problems, the Japanese Patent Application No. 10-351637 (hereinafter, referred to as a conventional example) discloses a method for manufacturing a color filter on a TFT (the TFT substrate provided with a Color Filter). FIGS. 1 and 2 are cross-sectional views of a TFT substrate provided with a CF in which a TFT is protected by a passivation film and used as a switching element, illustrating steps for manufacturing the TFT. The structure of a TFT provided with a CF will be described with reference to FIGS. 1 and 2.
First, a TFT 160 in which a channel thereof is formed by etching is formed on a transparent insulating substrate 51, and an entire surface of the substrate including the TFT 160 is covered by a passivation film 58. The passivation film 58 is formed by, for example, depositing a silicon nitride using plasma CVD (FIG. 1A).
Next, a negative type photocurable color resist obtained by dispersing a red pigment in an acryl resin is spin-coated on the transparent insulating substrate 51. The rotational speed of a spinner is adjusted to make a film thickness of the resist about 1.2 xcexcm. Then, the substrate 51 having the resist formed thereon is heated on a hot plate for two minutes at a temperature of 80xc2x0 C. in a pre-baking step, and exposed and further, developed in a TMAH solution (tetramethylammonium hydroxide) to form a red color filter 163 in an associated portion on the substrate 51 (FIG. 1B). In this case, the red color filter 163 is formed such that the red color filter is not formed on a part 62 of the passivation film 58 in which part a third opening will be formed later in the passivation film. Then, the substrate 51 is baked in a clean oven for 60 minutes at a temperature of 220xc2x0 C. to cure the red color filter 163.
Thereafter, a green color filter 263 is formed in another pixel, in which a color filter other than the red color filter is to be formed, in accordance with the same manner as in the case where the red color filter has been formed. The substrate 51 is baked in an oven for 60 minutes at a temperature of 220xc2x0 C. to obtain the green color filter 263. A blue color filter 363 is also formed in accordance with the same manner as in the case where the red color filter has been formed.
Subsequently, after completion of formation of color filters, a black matrix 64 is formed (FIG. 1C). The black matrix 64 is formed of a resin made of a carbon or a pigment dispersed in an acryl resin. For example, such a material having a viscosity of about 20 cp is spin-coated on the transparent insulating substrate 51 to have a film thickness of about 1.5 xcexcm and then, the material is developed. In this case, the black matrix is not formed on a portion of the substrate 51 in which a contact through hole will be formed in a later step.
An overcoat layer 65 is coated to flatten the surface of the substrate 51 and developed to have a first opening 66 therein. The substrate 51 is baked for 60 minutes at a temperature of 220 to 230xc2x0 C. to cure the overcoat layer 15. In this case, the overcoat layer is melted by the baking to have a cross sectional profile shaped like an arch having a large curvature (FIG. 2A).
Then, a novolac-type photoresist 67 is coated and patterned to have a second opening 68 therein. Thereafter, the passivation film 58 is etched using the novolac type photosensitive resist 67 as a mask, thereby forming a third opening 69 in the passivation film 58 (FIG. 2B)
After completion of formation of the overcoat layer 65 and third opening 69, the novolac-type photoresist 67 is removed and a transparent conductive film to serve as a pixel electrode is formed covering the aforementioned components by sputtering a transparent material, and then, the transparent conductive film is patterned to form a pixel electrode 70 (FIG. 2C). In this case, when the transparent conductive film is formed thicker, the pixel electrode 70 can more securely cover the associated portions to thereby achieve stable electrical connection between the pixel electrode 70 and the drain electrode 57. However, in consideration of ease of operation for processing an ITO (indium-tin-oxide) film, it is preferable to deposit the ITO (indium-tin-oxide) film to a film thickness of about 100 nm.
According to this conventional example, the novolac type photosensitive resist is coated on the overcoat layer shaped like an arch and patterned to form an opening in the passivation film through the patterned resist, so that the pixel electrode and the drain electrode can be connected through the opening. In this case, the second opening 68 is designed to include the first opening 66 therein while having 1 xcexcm alignment allowance with respect to the first opening 66. However, actually in an alignment step, the second opening 68 is occasionally positioned inside the first opening 66 owing to variation caused during manufacturing process. This phenomenon makes an inner wall surface of the second opening 68 of the novolac type photosensitive resist stand in a direction substantially vertical to the surface of the substrate 51 along the first opening 66 of the overcoat layer at an interface between the resist and the passivation film. Therefore, in this portion, a cross sectional profile of the third opening of the passivation film is substantially vertical to the surface of the substrate 51, contributing to degradation of cross sectional profile of the pixel electrode along the third opening 69 and then, unfavorably generating an unstable connection resistance between the pixel electrode and the drain electrode.
An object of the present invention is to provide a method for manufacturing an active matrix substrate, in more detail, a TFT substrate having a color filter thereon, the method allowing connection between a source/drain electrode and a pixel electrode connected thereto to stably become low.
A method for manufacturing an active matrix substrate constructed in accordance with the present invention can be performed in the following steps. That is, a thin film transistor and a wiring are formed on a transparent insulating substrate, and a protective film covering the thin film transistor and the wiring are formed on the transparent insulating substrate, and then, a first photosensitive film is formed on the protective film exposed therefrom, the first photosensitive film having a first opening to expose a portion of the first region of the protective film. Thereafter, a second photosensitive film is formed on the first photosensitive film with a second opening positioned inside the first opening to expose a part of the protective film and a third opening is formed in the protective film. In this case, the third opening is formed by removing the part of the protective film to expose a portion of the wiring by using the second photosensitive film as a mask such that an edge of the third opening is apart from an inner wall of the first opening, removing the second photosensitive film to expose the first photosensitive film by a distance equal to at least a film thickness of the second photosensitive film. Then, a conductive film is formed on the first photosensitive film so as to be connected to the wiring through the third opening and finally, a pattern is formed in the conductive film to form an upper layer wiring made of the conductive film.
As described above, in a case where the second opening is formed in the second photosensitive film to have a tapered cross sectional profile, the third opening also can be formed in the protective film to have a tapered cross sectional profile, allowing for stable connection between a wiring and an upper layer wiring connected to each other through the third opening.